1. Field of the Invention
The present invention relates to a process control apparatus for a PI or PID (P: proportional, I: integral, D: derivative) control arithmetic operation which is applicable to various control systems as commonly represented by a plant instrumentation and control system, and more particularly to a process control apparatus wherein a filter element depending on integration time of a PI or PID control arithmetic operation is inserted in a feedback line and both disturbance suppression characteristics and set point value following characteristics are simultaneously improved.
2. Description of the Related Art
A PI or PID arithmetic operation has widely been utilized in various industrial fields since the beginning of control technology. Indeed control systems employed in various industrial fields will not work without some type of PI or PID control arithmetic operation.
In a conventional PI or PID control apparatus, a PI or PID control arithmetic operation is performed on the basis of a deviation between a set point value and a process value output from an object to be controlled. Basic equations of the PI and PID control arithmetic operations are given by: ##EQU1## where
MV (s) is a manipulative value supplied to the object to be controlled,
E(s) is a deviation,
C(s) is a transfer function of a regulation portion at which the control arithmetic operation is performed to obtain the manipulative value,
K.sub.P : a proportional gain,
T.sub.I is an integral time,
T.sub.D is a derivative time,
s is Laplace's operator, and
1/.eta. is a derivative gain.
In the conventional process control apparatus, a control loop can be formed by the basic equation (1) or (2), as shown in FIG. 1. A set point value SV(s) is supplied to a deviation arithmetic operation circuit 2, and a deviation E(s) (=SV(s)-PV.sub.1 (s)) between the set point value SV(s) and a process value PV.sub.1 (s) from a control object 1 is obtained. The deviation E(s) is supplied to a control arithmetic operation circuit 3 for solving equation (1) or (2). A disturbance D(s) supplied from an external source is added to a manipulative value MV(s) from the control arithmetic operation circuit 3 by an adder 4. The result of this addition is supplied to the control object 1.
The process value PV.sub.1 (s) of the control apparatus as shown in FIG. 1 is expressed as follows: ##EQU2## where
C(s) is a transfer function of the control arithmetic operation circuit 3 (see equation (1) or 2)), and
P(s) is a transfer function of the control object 1.
Equation (3) is also called a "response equation" of the process value.
Regarding the right side terms of equation (3), the first term relates to the set point following characteristic, and the second term relates to the disturbance suppression characteristic. These terms have proper control constants (proportional gain K.sub.P, integral time T.sub.I, derivative time T.sub.D), However, since the control constants are related commonly to both terms, it is not possible to set optimal values to satisfy both characteristics at the same time. For example, if the control constants are selected to optimize the disturbance suppression characteristic, a great overshoot occurs in the set point following characteristic, and the set point following characteristic become vibrational. Inversely, if the set point value following characteristic are optimized, the disturbance suppression characteristic are degraded, and thus there is an antinomic relationship. This control apparatus is called a "control apparatus of one degree of freedom."
In order to solve this problem, "a control apparatus of two degrees of freedom" has been developed, wherein the set point value following characteristic and disturbance suppression characteristic are controlled independently. FIG. 2 shows a control apparatus wherein a set point value filter 5 of a transfer function H(s) for manipulating the set point value SV(s) is provided in front of the deviation arithmetic operation circuit 2 in the control apparatus of FIG. 1. The other structural points in FIG. 2 are identical to those in FIG. 1. A control value PV.sub.2 (s) of the control loop shown in FIG. 2 is expressed as follows: ##EQU3##
The difference between equations (4) and (3) is that only the numerator of the first term of equation (4) is multiplied by the transfer function H(s) of the set point value filter 5. Thus, the control constants of the control apparatus are set to optimize the factor due to disturbance in the second term, i.e. the disturbance suppression characteristic. In accordance with the set constant of the set point value filter 5, only the control constants associated with change of the set point value are automatically changed. Thereby, only the set point value following characteristic is improved without affecting the disturbance suppression characteristics of the second term. The transfer function H(s) of the set point value filter 5 needs to be set such that the gain=1 at steady state and&lt;1 at transition state, since the transfer function H(s) compensates the variation of the set point value and transition characteristic.
However, since there is an increasing demand for high-efficiency, flexible plant operations, there is a case where satisfactory disturbance suppression characteristic cannot be obtained by the conventional control system of two degrees of freedom. Considering the present situation in which PID or PI control apparatuses are employed in about 90% of plant control systems, an improvement of characteristics of two-degrees-of-freedom control apparatuses will be conducive to manufacturing of high-quality products.